IBOC which the FCC (Federal Communications Commission: Federal Communications Commission) has approved as standards, and which is a broadcast system for HD radio put to practical use in the U.S. uses a hybrid method of transmitting a digital signal simultaneously with an existing AM (Amplitude Modulation)/FM signal (Frequency Modulation) analog broadcast using the frequency of the analog broadcast.
In the above-mentioned HD broadcast, the same sound is broadcast fundamentally as both an analog sound and a digital sound, and a radio broadcast receiver that receives the hybrid broadcast outputs the digital sound when being able to receive the digital signal, and outputs the analog sound when being unable to receive the digital signal.
In this HD broadcast, the analog signal is broadcast while being delayed with respect to the digital signal by a delay time which is assumed to occur in a digital broadcast signal processing decoder. Therefore, a relatively long transition time is set up for switching between the analog sound and the digital sound in such a way that no time lag occurs between them and the user does not have a feeling that something is abnormal due to the difference in frequency characteristics between them.
By the way, a conventional broadcast receiver having a delay means for detecting a time lag between an FM-modulated analog sound and a digital sound (e.g., DAB: Digital Audio Broadcasting) to compensate for this time lag has been proposed. This broadcast receiver carries out a process of reducing an audible distortion which occurs when switching between frequencies or between broadcast types so as not to make the user have a feeling that something is abnormal (for example, refer to patent reference 1).